“American Idol” producers have a tricky way of keeping tabs on contestants looking to sneak out of their rooms to bang other contestants … according to former ‘Idol’ contestant Jackie Tohn.

We got Jackie, who was on season 8, Monday night at the Hollywood Improv, and she had a flashback when we told the new ‘Idol’ contestants have been hooking up like crazy.

Jackie says back in her day, producers put a simple piece of tape to good use. You gotta see her break down the strategy used to make sure ‘Idol’ hopefuls were working on music, and not each other. Hint: Don’t break the seal!

It doesn’t sound like she was having as much fun with her fellow costars as the kids are now, but Jackie does have a tip for how to keep their hotel romps under wraps.